A passive optical network (PON) is a point to multi-point (P2MP) fiber transmission and access network, in which simple optical branching devices (OBDs) instead of node equipment are required at optical branching points. The PON may be flexibly formed into a tree, star, bus, or other types of topological structure. FIG. 1 is a topological view of a PON in the conventional art. Referring to FIG. 1, the PON includes a central office side optical line terminal (OLT) 91, subscriber side optical network units (ONUs) 92 or optical network terminals (ONTs), and an optical distribution network (ODN) 93. The ODN does not include any active electronic devices or power sources, but includes optical splitters and other passive devices, so that the management and maintenance costs are low.
In the PON system, data is transmitted from the OLT 91 to the ONUs 92 in a downlink direction by using a time division multiplex (TDM) mode, that is, downlink data is continuously transmitted, the OLT 91 continuously sends information by broadcasting to each ONU 92, and each ONU 92 selectively receives the data corresponding thereto.
Moreover, data is transmitted from the ONUs 92 to the OLT 91 in an uplink direction by using a time division multiplex address (TDMA) mode, that is, uplink data is transmitted in a burst manner, different ONUs 92 occupy different uplink time slots, and the multiple ONUs 92 share the uplink in the TDM mode. A guard time is provided between adjacent uplink time slots to prevent collision.
To avoid collision of the uplink data, ranging needs to be implemented for the uplink transmission. A loop delay of data signals in a period receives data in an uplink manner is measured, where the period is from the moment that the OLT 91 sends data in a downlink manner to the moment that the OLT 91, and a delay compensation is performed according to the loop delay, to ensure that when assigned time slots are inserted after uplink signals of the ONUs 92 are gathered at public fibers, collision between the signals is prevented and slots in a proper size are obtained.
To utilize fiber resources of an existing gigabit Ethernet passive optical network (GEPON) system, and as the future 10G EPON needs to coexist with the GEPON system, that is, to form a 10G/1G EPON system, the OLT is required to support the ONUs at the two rates of 10G and 1G.
FIG. 2 shows a solution in which the 10G EPON and 1G EPON systems coexist. In the solution, the uplink transmission is implemented with the same wavelength by using the TDM mode, and the downlink transmission is implemented with different wavelengths by using a wave division multiplex (WDM) mode. Because the uplink transmission is implemented with the same wavelength by using the TDM mode, a multi-rate receiving apparatus (MR-RX) needs to be provided at the OLT to receive the 1G and 10G ONU uplink data in a time-division manner.
FIG. 3 is a schematic structural view of a receiving apparatus in the conventional art. A receiving end of the receiving apparatus adopts a 1G/8GHz transimpedance amplifier (TIA) 80 which has a variable bandwidth for receiving, and the bandwidth of the TIA 80 is changed by switching between resistance values of feedback resistors R1 81 and R2 82. For 10G signals, the feedback resistor is switched to R2 82, the bandwidth of the TIA 80 is 8 GHZ, and the TIA 80 is adapted to receive the 10-Gbit/s signals. For 1G signals, the feedback resistor is switched to R1 81, the bandwidth of the TIA 80 is changed from 8 GHz to 1 GHz, and the TIA 80 is adapted to receive the 1-Gbit/s signals, thereby preventing the bandwidth of the TIA 80 from getting excessively large to generate too many noises and affect the receiving sensitivity of the 1G signals when the 1G signals are received.
Nevertheless, the above receiving apparatus requires a dedicated 1G/8G TIA, and the structure of the 1G/8G TIA is complicated and difficult to realize.
FIG. 4 is a schematic structural view of another receiving apparatus in the conventional art. A receiving end of the receiving apparatus shares a module of a 2.5G photoelectric detector (PD)/TIA 70, and signals are split after passing through the PD/TIA 70. In a 1G path, a passive filter (PF) 71 is adopted to filter out high frequency components, and modules of a 1G automatic gain control (AGC) 72 and a clock and data recovery (CDR) 73 are then adopted to receive the signals. In a 10G path, the 2.5-GHz PD/TIA 70 increases the attenuation of high frequency (greater than 2.5 GHz) parts of the received signals, and the signals are distorted, so that an active finite impulse response (FIR) filter 76 is required to compensate the high frequency attenuation, thereby recovering the 10G signals, and then a 10G AGC 77 and a CDR 78 are adopted to receive the signals.
Nevertheless, in the 10G path, if the active FIR filter is adopted to compensate the bandwidth limit of the 2.5-GHz TIA, the signal quality after compensation is undesirable.